“If Biden is elected, he’s going to be 82 years old in four years and he won’t be running for reelection,” a prominent adviser to the campaign told Politico.
“I’m in pretty good health, but, look, I guarantee you this: If anything changed in my health, making it incapable for me to fully exert all the energy and mental acuity that was needed to be done, then I give you my word: I would not run again,” Biden said.
December 2019:
https://www.rollingstone.com/politics/politics-news/report-biden-signaling-hed-only-serve-one-term-925434/
By Feb 2020 that wording changed slightly:
https://www.washingtonexaminer.com/news/443910/biden-promises-one-term-presidency-if-his-energy-and-mental-acuity-decline/